This application pertains to the art of air conditioning methods and apparatus. More particularly, this application pertains to methods and apparatus for efficient control of the moisture content of an air stream which has undergone a cooling process as by flowing through an air conditioning cooling coil or the like. The invention is specifically applicable to dehumidification of a supply air flow into the occupied space of commercial or residential structures. By means of selective combination of extracted return air flow heat energy and recovered refrigerant waste heat energy, the supply air flow is warmed using a reheat coil apparatus. The return air flow entering the air conditioning coil is precooled with a precooling coil in operative fluid communication with the reheat coil. Heating of the occupied space may be effected using the combined reheat and precooling coils in conjunction with an alternative heat source such as electric, solar, or the like and will be described with particular reference thereto. It will be appreciated, though, that the invention has other and broader applications such as cyclic heating applications wherein a supply air flow is heated at the reheat coil irrespective of the instantaneous operational mode of the refrigerant system through the expedient of a thermal energy storage tank or the like.
Conventional air conditioning systems use a vapor compression refrigeration cycle that operates to cool an indoor air stream through the action of heat transfer as the air stream comes in close contact with evaporator type or flooded coil type refrigerant-to-air heat exchangers or coils. Cooling is accomplished by a reduction of temperature as an air stream passes through the cooling coil. This process is commonly referred to as sensible heat removal. A corresponding simultaneous reduction in the moisture content of the air stream typically also occurs to some extent and is known as latent heat removal or more generally called dehumidification. Usually the cooling itself is controlled by means of a thermostat or other apparatus in the occupied space which respond to changes in dry bulb temperature. When controlled in this manner, dehumidification occurs as a secondary effect incidental to the cooling process itself. As such, dehumidification of the indoor air occurs only when there is a demand for reduced temperature as dictated by the thermostat.
To accomplish dehumidification when the thermostat does not indicate a need for cooling, a humidistat is often added to actuate the air conditioning unit in order to remove moisture from the cooled air stream as a "byproduct" function of the cooling. In this mode of operation, heat must be selectively added to the cooled air stream to prevent the conditioned space from over-cooling below the dry bulb set point temperature. This practice is commonly known as "reheat".
Many sources of heat have been used for reheat purposes, such as hydronic hot water with various fuel sources, hydronic heat recovery sources, gas heat, hot gas or hot liquid refrigerant heat, and electric heat. Electric heat is most often used because it is usually the least expensive alternative overall. However, the use of electric heat to provide the reheat energy is proscribed by law in some states, including Florida for example.
In order to conserve energy, it has been suggested that recovered heat be used as a source for the reheat. Accordingly, one method to improve the moisture removal capacity of an air conditioning unit, while simultaneously providing reheat, is to provide two heat exchange surfaces each in one of the air streams entering or leaving the cooling coil while circulating a working fluid between the two heat exchangers. This type of simple system is commonly called a run-around system.
Run around systems have met with limited success. The working fluid is cooled in a first heat exchange surface placed in the supply air stream called a reheat coil. The cooled working fluid is then in turn caused to circulate through a second heat exchange surface placed in the return air stream called a precooling coil. This simple closed loop circuit comprises the typical run-around systems available heretofore.
The precooling coil serves to precool the return air flow prior to its entering the air conditioning cooling coil itself. The air conditioning coil then provides more of its cooling capacity for the removal of moisture from the air stream otherwise used for sensible cooling. However, the amount of reheat energy available in this process is approximately equal to the amount of precooling accomplished. This is a serious constraint. Additional reheat energy is often needed for injection into the run-around system to maintain the desired dry bulb set point temperature and humidity level in the conditioned space. As described above, supplemental electric reheat has been used with some success.
In addition, the growth of molds in low velocity air conditioning duct systems has recently become a major indoor air quality concern. One of the control measures recognized as having the capability of limiting this undesirable growth is the maintenance of the relative humidity at 70 percent or lower in the air conditioning system air plenums and ducts. Within limits, reheat can be used to precisely control the relative humidity. However, as described above, the amount of reheat energy from the run-around systems available today may not be sufficient to consistently provide the above level of humidity control, particular during periods of operation when the air temperature entering the precooling coil is lower than the system design operating temperature.
As a further complication, air conditioning units are also often used for heating purposes as well as for cooling and dehumidification. Electric heating elements are often provided in the air conditioning units to selectively provide the desired amount of heat at precise times of the heating demand. The above demand for heating energy will most often correspond with the demand for heating at other air conditioning units in the locality. This places a substantial and noticeable demand on the electrical power utility system in the community. In many areas, this peak demand has exceeded the capacity of the power system. The electric utility companies have responded with incentives encouraging their customers to temper their demand during regional peak demand periods. These incentives are often in the form of demand charges which encourage the customer to reduce their demand on the system at those times in order to avoid incremental costs in addition to the regular base rates.
It has, therefore, been deemed desirable to provide an economical solution that meets the various needs of air conditioning system installation requirements while also operating in compliance with current and projected local environmental and energy related laws.